Abstract
Metal-enhanced fluorescence (MEF), resulting from the near-field interaction of fluorophores with metallic nanostructures, has emerged as a powerful tool for dramatically improving the performance of fluorescence-based biomedical applications. Allowing for lower autofluorescence and minimal photoinduced damage, the development of multifunctional and multiplexed MEF platforms in the near-infrared (NIR) windows is particularly desirable. Here, a low-cost fabrication method based on nanosphere lithography is applied to produce tunable three-dimensional (3D) gold (Au) nanohole-disc arrays (Au-NHDAs). The arrays consist of nanoscale glass pillars atop nanoholes in a Au thin film: the top surfaces of the pillars are Au-covered (effectively nanodiscs), and small Au nanoparticles (nanodots) are located on the sidewalls of the pillars. This 3D hole-disc (and possibly nanodot) construct is critical to the properties of the device. The versatility of our approach is illustrated through the production of uniform and highly reproducible Au-NHDAs with controlled structural properties and tunable optical features in the NIR windows. Au-NHDAs allow for a very large NIR fluorescence enhancement (more than 400 times), which is attributed to the 3D plasmonic structure of the arrays that allows strong surface plasmon polariton and localized surface plasmon resonance coupling through glass nanogaps. By considering arrays with the same resonance peak and the same nanodisc separation distance, we show that the enhancement factor varies with nanodisc diameter. Using computational electromagnetic modeling, the electric field enhancement at 790 nm was calculated to provide insights into excitation enhancement, which occurs due to an increase in the intensity of the electric field. Fluorescence lifetime measurements indicate that the total fluorescence enhancement may depend on controlling excitation enhancement and therefore the array morphology. Our findings provide important insights into the mechanism of MEF from 3D plasmonic arrays and establish a low-cost versatile approach that could pave the way for novel NIR-MEF bioapplications.
Highlights
The large near-infrared fluorescence enhancement measured for Au-NHDAs fabricated by nanosphere lithography confirms that these arrays are promising NIR-Metal-enhanced fluorescence (MEF) platforms for the development of biosensing applications, where analytes at extremely low concentrations can be detected
By considering arrays with the same pitch and resonance peak, we showed that emission enhancement was not significantly different for the nanodisc diameters considered
These findings show that the enhancement factors, and the sensitivity of Au-NHDAs, can be tuned by manipulating the structural characteristics of the arrays, highlighting the potential of our fabrication protocol, which is flexible to several different structures
Summary
Metal-enhanced fluorescence (MEF) is an optical process in which the near-field interaction of fluorophores with metallic nanoparticles can, under certain conditions, lead to large fluorescence enhancement.[1−4] MEF has attracted considerable interest for fluorescence-based biomedical applications,[1] such as DNA5,6 and RNA7 sensing, immunoassays,[8,9] or fluorescence-based imaging.[10−12] For such bioapplications, fluorophores emitting in the near-infrared (NIR: 650−900 nm) and second near-infrared (NIR-II: 1.0− 1.7 μm) windows are of particular importance,[13−15] since low absorption of light by water and hemoglobin at these wavelengths allows high transparency for potential tissue imaging.[16]. This enhancement has already been shown to dramatically improve the sensitivity and dynamic range for protein biomarker detection in fluorescent immunoassays.[26] Here, the tunability of the enhancement factors was achieved, with higher fluorescence enhancement measured for Au-NHDA-215 compared to that for Au-NHDA-148 We believe this is likely due to the shorter distance between individual nanodiscs leading to stronger LSPR interparticle coupling, which is consistent with the previously reported work.[4,23,24] the protocol presented here for Au-NHDA fabrication may be a significant step forward in the application of MEF in biomedical applications as it allows straightforward control of the arrays’ structural properties, in contrast to nanoimprint lithography, which lacks the flexibility in producing different structures using the same mask. Further lifetime data may be required to more thoroughly characterize our system, our findings indicate that array morphology may significantly impact the mechanism of fluorescence enhancement, emphasizing the capabilities of our fabrication protocol in producing arrays with controlled structural properties to achieve tunable enhancement factors
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